portage_util: Detect ebuild multilib_src_test()

Mark ebuilds with multilib_src_test() as containing tests so
that cros_run_unit_tests can be used on them.

BUG=chromium:1001398
TEST=cros_run_unit_tests --board betty --packages cups

Change-Id: I56407505641a08e6e001b7c610c0b82674e291d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2370413
Tested-by: Pranav Batra <batrapranav@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Pranav Batra <batrapranav@chromium.org>
diff --git a/lib/portage_util.py b/lib/portage_util.py
index a8f8189..353908d 100644
--- a/lib/portage_util.py
+++ b/lib/portage_util.py
@@ -561,7 +561,8 @@
         elif line.startswith('CROS_WORKON_BLACKLIST='):
           is_blacklisted = True
         elif (line.startswith('src_test()') or
-              line.startswith('platform_pkg_test()')):
+              line.startswith('platform_pkg_test()') or
+              line.startswith('multilib_src_test()')):
           has_test = True
         elif line.startswith('RESTRICT=') and 'test' in line:
           restrict_tests = True